This invention relates to pneumatic control valves and, in particular, to valves for controlling the air flow to the blades of circulation control helicopter rotors.
Conventional helicopter blades produce strong vibrations because of the widely varying amounts of lift that are produced by different parts of the blade as it rotates. The conventional ways of dealing with these vibrations have included making the helicopter rotors very flexible and varying the angle of attack of each blade to regulate the amount of lift produced by the blade. Circulation control helicopter rotors, such as those disclosed in U.S. Pat. No. 3,713,750, have rigid blades with a fixed angle of attack. The blades on a circulation control rotor (CCR) have thin slots on the edges through which a stream of pressurized air is ejected to control the lift characteristics of the blades. By carefully controlling the air pressure which is supplied to the blowing slots on the edges of the blades, the blades of a circulation control rotor can be made to produce a constant lift regardless of the angular position of the blades, the angular velocity of the blades, or the forward speed of the helicopter. To obtain this constant lift from a circulation control helicopter blade, it is necessary to regulate the air pressure being fed to the slots on the edges of the blade as a function of the azimuth angle between the direction the helicopter is flying and the long axis of the rotating blade. This air pressure will have a periodic waveform which includes a fundamental frequency the same as the rate of rotation of the helicopter rotor plus several harmonics of this fundamental frequency.
The prior art discloses several valves which may be used to generate these air pressure waveforms. One example of such a valve system is disclosed in U.S. Pat. No. 3,816,019, which uses stationary cams to restrict the flow of pressurized air from an air plenum inside the rotor hub into the rotor blades. The leading edge slots and trailing edge slots on a circulation control rotor could be provided with independent air pressure waveforms by placing two independent valve systems of this kind within the rotor hub. The air pressure waveforms produced by the prior art valves are limited to combinations of a fundamental sine wave plus a few of its harmonics. The prior art valves cannot produce air pressure waveforms with rise times and fall times which are a small percentage of the basic time period of the waveform. Neither are the prior art valves able to periodically switch the flow of pressurized air on and off during each rotation of the helicopter rotor.
Research done by the inventors has shown that the lack of these capabilities in the prior art valves for circulation control rotors will limit any helicopter which uses these prior art valves to the relatively low forward velocities at which conventional helicopters presently operate. The inventors have shown that a helicopter with a circulation control rotor having blowing slots on both the leading and trailing edges, together with an appropriate valve system, could be made to fly at forward velocities which are much higher than is possible with conventional helicopters. FIGS. 3e, 3f, and 3g illustrates the kind of air pressure waveforms which the valve system would be required to generate for such a high speed helicopter. The advance ratio of a helicopter is the forward velocity of the helicopter divided by the velocity of the tip of its rotor blades. FIG. 3e, illustrates the air pressure waveforms which would be required for the leading edge and trailing edge slots on the blades of a circulation control helicopter rotor when the advance ratio is approximately 0.5 or less. In this situation the trailing edge slots are provided with an air pressure waveform which is a simple sine wave, and the leading edge slots are provided with essentially no air pressure at all. FIG. 3f, illustrates typical air pressure waveforms which would be required for the blades of a circulation control helicopter rotor for the transition advanced ratios between 0.5 and 1.4. In this case the waveforms for both the trailing edge and leading edge slots have the same fundamental frequency plus several harmonics. However, the number of harmonics included in the leading edge waveform is much greater and the leading edge waveform has very fast rise and fall times. When the advanced ratio of the helicopter is greater than 1.4, the kind of air pressure waveforms required for the leading and trailing edges will be like those shown in FIG. 3g, have very fast rise times and fall times. A high speed helicopter with a circulation control rotor must have a valve system capable of changing its waveforms between the types shown in FIGS. 3e, 3f and 3g while the helicopter is in flight.
Most of the prior art control valves for circulation control rotors as well as the valve disclosed in this specification are intended to be mounted inside of the rotating hub of the helicopter rotor. Many of the prior art valves require several moving parts in addition to the rotating shell of the rotor hub. These reciprocating parts must repeat their motions at least once for every rotation of the rotor in order to generate the proper air pressure waveforms. Examples of such valve systems are those disclosed in U.S. Pat. Nos. 3,904,313, and 3,918,833.